As described in U.S. Pat. No. 5,424,625, for example, the BLR motor is constructed almost like a universal motor used in hand drills, etc. Its stator consists of one or more salient poles whose copper windings are connected directly to the two legs of a single phase, AC line, just like a universal motor. A BLR motor with three pairs of stator poles runs on three-phase power. The armature consists of slotted steel laminations stacked on a shaft. Copper wire is wound into each slot, over the stack end and back into the slot on the opposite side. The number of turns and the wire size vary with the performance desired. The two ends of each coil are connected by a switch, such as a triac or pair of transistors, thus forming an electric circuit.
When the stator windings are connected to an AC line, magnetic flux builds up and collapses with the line current. This flux passes directly through the armature and induces a voltage potential on each armature coil. When a coil's switch is closed, the current flows in that coil as a result of the applied voltage. This produces opposing magnetic flux and thus torque and rotation. When the switch is open current cannot flow and no torque is produced.
In a two-pole BLR motor, torque is produced in a clockwise direction when a coil is “on” in a 90 degree sector on one side of a stator pole (the positive torque sector). When turned on in the negative torque sector on the other side of the pole, torque is produced in the opposite direction. So, in a two-pole motor, each coil passes through two positive and two negative torque sectors per revolution. Of course, only the positive torque or the negative torque sectors are activated at any one time.
The switches can be opened or closed at will by stationary signal means. The signals can be RF, magnetic, sonic, light, etc. For example, an inexpensive and reliable means is a curved array of infrared light emitting diodes (LEDs) mounted on the motor end-bell. These can be illuminated individually or together. On the rotating armature there is a photo-detector associated with every coil. When a detector “sees” an illuminated LED it closes its switch, which produces current in the coil and flux and torque. By lighting the LEDs in the negative torque sector, reverse rotation can be achieved.
When rotating clockwise, lighting only a single LED when the coil has almost completed its arc in the positive torque sector produces little torque and speed. By lighting the entire array, each switch is turned on during its full arc and therefore develops maximum power and speed. Current in an armature coil is highest when the coil is aligned with the stator pole and cuts the maximum number of flux lines. It falls to zero as the coil rotates 90 degrees and leaves that sector. Therefore, switches are generally turned off at or near the end of a sector in order to break minimum current and achieve the maximum efficiency.
However, when the coil is thus aligned, all the force is directed along the line from one pole to the other and produces no rotational torque. As the coil rotates it begins to produce torque and reaches its most effective torque producing position at 90 degrees, exactly where the current is zero. The actual torque produced at each rotational angle by the combination of these phenomena and other factors is an asymmetrical curve. It rises sharply from zero at the high current-low torque position (hard neutral) to a peak and then decreases more gradually as it moves 90 degrees toward the end of the positive torque sector (soft neutral), where it again becomes zero.
At higher speeds the dynamic interaction between the rotating and stationary magnetic fields tends to shift both the optimum turn-on and turn-off points. As a result, the positive torque sector extends to beyond the 90 degree static limit. This means that a lighted LED at the end of a sector may, under different conditions, produce either positive or negative torque. Therefore, to produce optimum torque and efficiency a speed control scheme must have the flexibility to handle this shift.
Thus, there is a need for a system that improves the performance, reliability and cost of a speed-controlled BLR motor. Such improvements include an internal speed and position sensor, mechanical refinements and electronic control means based on timing.